1. Field of the Invention
The present invention relates to a liquid ejection head for ejecting ink droplets, an inkjet printing apparatus and a liquid ejecting method, and particularly relates to enhancement of the durability of a liquid ejection head.
2. Description of the Related Art
Ink ejecting methods applied to generally-used inkjet printing apparatuses include a method for ejecting ink droplets by using a liquid ejection head in which heat-generating elements, such as heaters, are arranged as ejection-energy generating elements. In this method, first, ink around a heat-generating element is instantaneously boiled by applying a voltage to an electrothermal transducing element functioning as the heat generating element. A phase change of the ink at the time of boiling creates an abrupt increase of pressure, so that ink droplets are ejected from the liquid ejection head. By ejecting ink droplets in this manner, the inkjet printing apparatus can finely control the ejection of ink droplets in response to an electric signal.
An ink ejecting method using the heat-generating elements, such as electrothermal transducing elements, has advantages that a large space is not needed to arrange the ejection-energy generating elements; the structure of the printing head is simple; and thus a large number of nozzles can be easily arranged in a smaller space, for example. For these reasons, a growing number of inkjet printing apparatuses using this ink ejecting method have been in use recently.
However, in a case where printing is performed by the ink ejecting method, the pressure of ink may abruptly change and induce cavitation upon bursting of a bubble made in the ink by the heat-generating element. If this abrupt pressure change occurs around any of the heat-generating elements, it is likely to make an impact on the heat-generating element. The impact adversely affects the durability of the heat-generating element. Methods have been proposed for preventing such an abrupt pressure change from deteriorating the durability of heat-generating elements, and one of the methods is to print with a printing head disclosed, for example, in Japanese Patent Application Publication No. Hei. 11-188870.
Japanese Patent Application Publication No. Hei. 11-188870 discloses a printing head which causes bubbles and the atmosphere to communicate with each other once the bubbles start to reduce their volume. In the case where printing is performed by ejecting ink droplets from the printing head disclosed in Japanese Patent Application Publication No. Hei. 11-188870, a portion of ink which immediately follows each ejected main droplet of ink has a component which tends to shrink toward the heat-generating element. This facilitates separation of the main droplet from the portion of ink which would turn into a satellite droplet if ejected. Accordingly, this mechanism makes it possible to separate satellite droplets in case the ink ejection is performed, from the main droplets, thereby checking the occurrence of the satellite droplets. Thus, the occurrence of the satellite droplets which are separated from main droplets is prevented and prevents occurrence of a mist of ink floating between the printing apparatus and the printing medium.
In general, in the printing head which causes bubbles and the atmosphere to communicate with each other in the process of growth and shrinkage of the bubbles, gas forming each bubble is discharged to the outside when the bubble and the atmosphere communicate with each other. As a result, once the bubble disappears, the amount of gas existing in the liquid decreases. This inhibits an abrupt change in pressure in the liquid, and accordingly enhances the durability of the heaters.
However, even if the printing head which causes bubbles and the atmosphere to communicate with each other in the process of growth and shrinkage of the bubbles is used, a bubble is sometimes left in the liquid after the liquid droplet is ejected, so that the bubble abruptly changes the pressure inside the bubbling chamber when it bursts.
FIG. 10A is a cross-sectional view of a nozzle in a printing head of a conventional atmosphere-communication type. The nozzle is viewed in the ejection direction. FIG. 10B is a cross-sectional view of the nozzle, taken along the line B-B of FIG. 10A. In the printing head in which bubbles and the atmosphere communicate with each other when the bubbles shrink, a bubble communicates with the atmosphere by contacting the meniscus which moves toward the heat-generating element when the liquid droplet is ejected. At this time, the meniscus moves almost symmetrically with respect to an axis perpendicularly passing the center of the heat-generation element and keeps its shape symmetrical. By contrast, the shape of the bubble is partially asymmetrical because of the shape of the nozzle. Because the ink passage extends toward the ink supplying port, there is no wall surface restricting the shape of the bubble in that direction. However, there is a wall surface forming the bubbling chamber in the far-end portion thereof at the side opposite to the ink supplying port. The wall surface located in the far-end portion of the bubbling chamber restricts the growth of the bubble. As a result, a part of a bubble located at the ink supplying port side in the bubbling chamber partially has a different shape from a part of the bubble located at the far-end portion opposite to the ink supplying port side. In sum, at the ink supplying port side in the bubbling chamber, the part of the bubble grows larger without having any restriction, thus having a relatively large grown part. In contrast, at the far-end portion of the bubbling chamber, the bubble has a relatively small grown part because the wall surface forming the bubbling chamber restricts the growth of the part of the bubble there.
When the liquid droplet is ejected with the bubble enlarging in this manner, then the meniscus moves toward the heat-generating element. In this situation, it is likely that, as shown in FIG. 10B, the atmosphere may communicate with the part of the bubble at the ink supplying port side, whereas the atmosphere may not communicate with the part of the bubble at the far-end portion. As a result, a split part of the bubble not communicating with the atmosphere is likely to remain at the far-end portion of the bubbling chamber. Furthermore, an abrupt pressure change may occur in the liquid existing inside the bubbling chamber when this split part of the bubble disappears, and accordingly an impact may be directed against the heat-generating element.